Since the liquid crystal display utilized as a display device such as a liquid crystal monitor or a liquid crystal television device does not emit light by itself, it requires a lighting device such as a backlight device. As a light source for such a backlight device, a discharge lamp such as a cold cathode lamp is widely used. A high AC voltage necessary for lighting such a discharge lamp is usually obtained by boosting the output of an inverter circuit by a high voltage transformer.
Recently, a discharge lamp lighting apparatus has been proposed that has a series resonant circuit formed on the secondary side of a high voltage transformer and that has an H-bridge circuit for driving the primary side of the high voltage transformer at a frequency which is lower than the resonance frequency of the series resonant circuit, and at which a phase difference between voltage and current on the primary side of the high voltage transformer lies within a predetermined range from a minimum value (refer to Document Paper 1 for example).
FIG. 5 is a circuit block diagram showing such a discharge lamp lighting apparatus. In the discharge lamp lighting apparatus 100 shown in FIG. 5, on the secondary side of a high voltage transformer 101, a series resonant circuit is configured by a leakage inductance of the high voltage transformer 101, capacitors 131 and 132, and a parasitic capacitance 103 of a discharge lamp 109. The operating frequency of an H-bridge circuit 117 for driving the primary side of the high voltage transformer 101 is set to a frequency which is lower than the resonance frequency of this series resonant circuit, and at which the phase difference θ between voltage and current on the primary side of the high voltage transformer 101 lies within a predetermined range from a minimum value, whereby the high voltage transformer 101 attains enhanced power efficiency.
Here, the capacitors 131 and 132 connected to the secondary side of the high voltage transformer 101 function as auxiliary capacitances for the parasitic capacitance 103. By changing the capacitances of capacitors 131 and 132, the resonance frequency of the series resonance circuit formed on the secondary side can be set to a desired value. The capacitors 131 and 132 function also as voltage detecting means when the secondary side is open. A signal 133 of which the voltage has been divided by the capacitors 131 and 132 is inputted into an error amplifier 151 for voltage feedback, and an output voltage 152 from the error amplifier 151 is inputted into a protection circuit 150 and a PWM circuit 108. The protection circuit 150, when the output voltage 152 of the error amplifier 151 exceeds a predetermined threshold value, stops the operation of a logic circuit 129 to thereby prevent an overcurrent into the discharge lamp 109. To the discharge lamp 109, a current-voltage conversion circuit 110 for converting a tube current of the discharge lamp 109 is connected. An output voltage 109a of the discharge lamp 109 is inputted into an error amplifier 111, which outputs an output voltage 112 in accordance with a current of the discharge lamp 109 to the PWM circuit 108, whereby constant current control on the basis of pulse width modulation is performed.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2005-038683